1. Field of the Invention
The invention relates generally to data detection methods and apparatus, and more particularly to methods and apparatus for partial-response maximum-likelihood (PRML), extended partial-response maximum-likelihood (EPRML), and Viterbi data detection in a direct access storage device (DASD).
2. Description of the Prior Art
Partial-response signaling with maximum-likelihood sequence detection techniques are known for digital data communication and recording applications. Achievement of high-data density and high-data rates has resulted in the use of a PPML channel for writing and reading digital data on the disks.
Known commercial disk drives which include a PRML channel benefit from the fact that, with proper choice of the data rate, binary partial-response class-4 (PR4) signaling with maximum-likelihood sequence detection (MLSD) or PRML provides nearly optimal performance at the presently used linear recording densities. Typically magnetic recording channels operate with 0.8T/R&lt;p.sub.w50 &lt;1.6T/R where T is the channel encoded bit period, R is the code rate and p.sub.w50 is the width at the 50%-level of the channel's step response. For example, p.sub.w50 =(.beta..sub.user /(.pi.R))T where .beta. user represents normalized user data rate and R is the code rate specific to each scheme, for example, such as, PRML advantageously uses R=8/9.
The performance loss of PRML with digital filter equalization caused by noise enhancement due to the equalizing filter becomes increasingly significant when the channel operates at linear recording densities such as p.sub.w50 &gt;1.6T/R. As a consequence, PMRL may fail to meet product specifications at greater linear recording densities.
To increase area storage density, mainly by means of increasing the linear density, requires that the PRML channel be replaced or complemented with a more powerful scheme in order to meet competitive product specifications. However, development and implementation of an entirely new channel architecture is a complex and costly task whose scope contradicts today's requirement for cost-effective and quick-to-market solutions.
U.S. Pat. No. 4,786,890 discloses a class-IV PRML channel using a run-length limited (RLL) code. The disclosed class-IV partial response channel polynomial equals (1-D.sup.2), where D is a one-bit interval delay operator and D.sup.2 is a delay of two-bit interval delay operator and the channel response output waveform is described by taking the input waveform and subtracting from it the same waveform delayed by a two-bit interval. A (0,k=3/k1=5) PRML modulation code is utilized to encode 8 bit binary data into codewords comprised of 9 bit code sequences, where the maximum number k of consecutive zeroes allowed within a code sequence is 3 and the maximum number k1 of consecutive zeroes in the all-even or all-odd sequences is 5.
U.S. Pat. No. 5,196,849 discloses rate 8/9 block codes having maximum ones and run length constraints for use in a class-IV PRML channel.
Trellis coding techniques are used to provide a coding gain required in noisy or otherwise degraded channels. U.S. Pat. Nos. 4,888,775 and 4,888,779 describe trellis codes for PRML channels which provide significantly improved coding gains for transmission of digital data over PRML channels.
U.S. Pat. No. 4,609,907 discloses a method for bandwidth compression using partial response and run length limited coding. A first 1-D.sup.2 channel is used for detection of data with a 1+D channel for clocking.
A conventional EPRML channel design including extended (EPR4) equalization, timing and gain control represents a large jump in complexity as compared to a PRML channel. By conventional implementation methods, PRML and EPRML share very few common functional blocks. The conventional approach is considered unacceptable from a size, power and speed viewpoint. For EPRML, the calculations required for the 5-level gain and timing loops are more complex and slower. Also, the 5-level timing gradient calculation is considered to be less robust than the 3-level calculation for PRML. EPRML requires an 8-state non-interleaved Viterbi detector which by conventional implementation methods is not acceptable from a size, power and speed viewpoint. It is desirable to provide an EPRML implementation that allows for an acceptable size, cost and power to be achieved.
With a goal of increased linear density, it is desirable to implement an EPRML/PRML combination system to provide optimal performance over the entire disk radius.